Sahelanthropus tchadensis is a name of
a fossil that is dated to
about 7 million years ago. Its exact position—if
the fossil can be regarded as part of the hominidae tree—is not
made clear but there are arguments supporting and denying its place
in the hominid tree. Another complication in its classification and
understanding is because it is one of the earliest dated hominid
fossils and very few or if not any bone evidence other than the
partial cranium. It is dated about 3 million years
earlier than the Australopithecines, earliest of which
are dated to about 4 million years ago.

Contents

Fossils

Location of discovery

Detail of map

Restoration of the face of Sahelanthropus

Existing fossils – a relatively small cranium nicknamed
Toumaï ("hope of life" in the local Dazaga
language of Chad), five pieces of jaw and some teeth – make up a head that has a mixture
of derived and primitive features. The braincase, being only
340 cm³ to 360 cm³ in volume is similar to that of extant
chimpanzees and is notably less than the approximate human volume
of 1350 cm³. The teeth, brow ridges, and facial structure
differ markedly from those found in Homo sapiens. Due
to the distortion that the cranium has suffered, a 3D computer
reconstruction has not been produced.

Since no postcranial remains (bones below the skull) have been
discovered, it is as of yet unknown whether Sahelanthropus
tchadensis was indeed bipedal, although claims for an
anteriorly placed foramen magnum suggests that this may
have been the case, some paleontologists have disputed this
interpretation of the basicranium. Its canine wear is similar to
other Miocene apes.[2]
Moreover, according to recent information, the femur of an hominid
may have been discovered alongside the cranium but never
published.[3]

The fossils were discovered in the Djurab desert of Chad by a team of four lead by Michel Brunet; three
Chadians, Adoum Mahamat, Djimdoumalbaye Ahounta and Gongdibé
Fanoné, and Frenchman, Alain Beauvilain.[4] ·[5] All
known material of Sahelanthropus were found between July
2001 to March 2002 at three sites (TM 247, TM 266 which yielded
most of the material, and TM 292). The discoverers claimed that
S. tchadensis is the oldest known human ancestor after the
split of the human line from that of chimpanzees. The bones were found far from
most previous hominin fossil finds, which are from Eastern and Southern Africa. However, an Australopithecus
bahrelghazali mandible was found in Chad by Beauvilain A.,
Brunet M. and Moutaye A.H.E. as early as 1995.[6]

Relationship to
modern humans and great apes

Sahelanthropus may represent a common ancestor
of humans and chimpanzees; no consensus has been reached yet by the
scientific community. The original placement of this species as a
human ancestor but not a chimpanzee ancestor would complicate the
picture of human phylogeny. In particular, if Toumaï is a
direct human ancestor, then its facial features bring the status of
Australopithecus into doubt
because its thickened brow ridges were reported to be similar to
those of some later fossil hominids (notably Homo erectus), whereas
this morphology differs from that observed in all
australopithecines, most fossil hominids and extant humans.

Another possibility is that Toumaï is related to both humans and
chimpanzees, but is the ancestor of neither. Brigitte Senut and
Martin Pickford, the discoverers of Orrorin
tugenensis, suggested that the features of S.
tchadensis are consistent with a female proto-gorilla. Even if this claim is
upheld, then the find would lose none of its significance, for at
present precious few chimpanzee or gorilla ancestors have been
found anywhere in Africa. Thus if S. tchadensis is an
ancestral relative of the chimpanzees (or gorillas) then it
represents the first known member of their lineage.
Furthermore, S. tchadensis does indicate that the last
common ancestor of humans and chimpanzees is unlikely to resemble
chimpanzees very much, as had been previously supposed by some
paleontologists.[7] ·[8]
Unfortunately, the exact age of the fossil is somewhat hard to
determine. While molecular clocks are increasingly found
to be far more unreliable than initially believed[9]sedimentisotope
analysis which yielded an age of about 7 million years is
generally considered quite reliable. In this case however, the
fossils were found exposed in loose sand; co-discoverer Beauvilain
cautions that such sediment can be easily moved by the wind, unlike
packed earth.[10] In
fact, Toumaï was probably reburied in the recent past. Taphonomic
analysis reveals the likelihood of one, perhaps two, burial(s)
which seemingly occurred after the introduction of Islam in the
region. Two other hominid fossils (a left femur and a mandible)
were in the same “grave” along with various mammal remains.The
sediment surrounding the fossils might thus not be the material the
bones were originally deposited in, making it necessary to
corroborate the fossil's age by some other means.[11] The
fauna found at the site – namely
the anthracotheriidLibycosaurus
petrochii and the suid Nyanzachoerus syrticus – suggests an
age of more than 6 million years, as these species were probably extinct already by that
time.[12]

The split of the line into humans and chimpanzees (known as human-chimpanzee divergence) probably happened between 6.3 and 5.4 million years ago. This can be seen from genetic data.[1] Because the fossil is older than this spilt, its status is unclear. The first fossil found is known as Toumaï today.

Contents

Fossils

Some fossils, a cranium (skull), five pieces of jaw, and some teeth, make up a head that has features that are like both modern and primitive humans. The braincase is only 340 cm³ to 360 cm³ in volume, about the same as chimpanzees. It is much less than the human volume of about 1350 cm³.

The teeth and structure of the face are very different from Homo sapiens. The cranium found is damaged, it is very distorted, so no 3D computer reconstruction has been made. There are no bones other than parts of the skull. It is unknown if Sahelanthropus tchadensis was bipedal (walked on two legs). The placement of the foramen magnum suggests they were, though. Its canine wear is similar to other Miocene apes.

Relationship to modern humans and great apes

Sahelanthropus may be an ancestor of both humans and chimpanzees; no consensus has been reached yet by the scientific community. The difficulty is caused by its "mosaic of primitive and derived features".[2] Probably it lived in semi-open woodland and savannah, rather than the rainforest of present-day apes.[3]

Another possibility is that Toumaï is related to both humans and chimpanzees, but is the ancestor of neither. Brigitte Senut and Martin Pickford, the discoverers of Orrorin tugenensis, suggested that the features of S. tchadensis are consistent with a female proto-gorilla. Even if this claim is upheld, the find would still be significant, for at present few chimpanzee or gorilla ancestors have been found anywhere in Africa. Thus, if S. tchadensis is an ancestral relative of the chimpanzees (or gorillas), it represents the first known member of their lineage.

Furthermore, S. tchadensis does indicate that the last common ancestor of humans and chimpanzees is unlikely to resemble chimpanzees very much, as had been previously supposed by some paleontologists.[2][3]

The fauna found at the site suggest an age of more than 6 million years, as these species were probably extinct already by that time.[4]